The present invention relates to a cathode-ray tube apparatus for displaying color pictures or monochrome pictures by deflecting a plurality of electron beams or a single electron beam emitted from an electron gun by means of a deflecting yoke, and more particularly to a cathode-ray tube apparatus compensating the difference of distortion between the intermediate portion and the peripheral portions of the display screen.
Generally, the color cathode-ray tube apparatus comprises a vacuum envelope having a substantially rectangular panel on whose inner surface a phosphor screen is formed, a funnel, and a cylindrical neck, and it displays color pictures by deflecting a plurality of electron beams emitted from an electron gun arranged in the neck by means of a deflecting yoke disposed from the outer surface of the reduced-diameter portion of the funnel to the neck, and by scanning the phosphor screen through a shadow mask horizontally and vertically. Further, in the monochrome cathode-ray tube apparatus, the electron gun emits a single electron beam, and the shadow mask of the color cathode-ray tube apparatus is omitted therefrom.
In such a cathode-ray tube apparatus, the traveling length of the electron beams deflected to the diagonal ends of the phosphor screen is the longest, and they pass the nearest position from the deflecting coils of the deflecting yoke. Thus, the raster which should be essentially rectangular is distorted to a pin cushion shape. Usually, in order to correct this distortion, magnets having a polarity of assisting a vertical deflection are disposed on and under the opening of the phosphor screen side of the deflecting yoke, and compensates the distortion on the upper and lower ends of the raster (hereinafter, the distortion is called as North-South distortion, i.e., NS distortion). Further, the distortion on the left and right ends of the raster (hereinafter, the distortion is called as East-West distortion, i.e., EW distortion) is compensated by changing the horizontal deflecting voltage dynamically.
However, since the correction of distortion by means of magnets is weaker in the vertically intermediate portion compared with the upper and lower peripheral portions of the picture, there is a distortion called "Inner Kissen Distortion" in the intermediate portion. As a result, there is a difference of distortion between the vertically intermediate portion and the peripheral portion of the picture.
This difference of distortion has been noticeable in recent wide deflection angle color cathode-ray tube apparatus and color cathode-ray tube apparatus having a flat screen.